Microencapsulation is the envelopment of an active agent or a core material within a solid coating. The active or core material can be in the form of a solid particle, a liquid droplet, or a gas bubble. The solid coating used to form the capsule may be, for example, an organic polymer, a wax, or an inorganic oxide. A capsule is characterized in general by parameters such as particle size and distribution, particle geometry, active contents and distribution, release mechanism, and storage stability.
Many encapsulation processes have been reported in the literature; only a few, however, have been commercialized. These include interfacial and in-situ polymerization, complex cocervation, spray drying, and fluidized-bed coating (the Wurster process). Others are used in low volume specialty applications. Interfacial polymerization is by far the most successful commercial process.
Microcapsule-based products are used in the graphic arts, adhesives, pharmaceutical, food, and pesticide industries. Carbonless copy paper is by far the largest use for microcapsules. Microcapsules containing solvents, liquid epoxy, or acrylate monomers are also manufactured commercially and used in adhesive formulations.
The patent literature has described imaging systems that utilize microcapsules as the key component for developability and color/tone scale differentiation by heat or pressure. These systems are very useful as they do not use conventional photographic wet processing. Heat or pressure developable photographic products, such as Thermo-Autochrome (Fuji Photo Film Co. Ltd.) and Cycolor Dry Media (Cycolor Inc.), have been commercially available.
Microcapsules described in the art for use in imaging applications are almost exclusively prepared by interfacial and in-situ polymerization processes. In interfacial polymerization, the materials used to form the capsule wall are in separate phases, one in the aqueous phase and the other in the oil phase. Polymerization occurs at the phase boundary. Wall formation of polyester, polyamides, and polyurea proceeds by interfacial polymerization.
Polyurea capsule walls can also be made by dissolving a polyisocyanate adduct in the oil phase. Hydrolysis of the isocyanate groups at the phase boundary form amine groups that in turn react with isocyanate groups to form urea linkages. In in-situ polymerization, the capsule wall forming materials are dissolved in the aqueous phase as resin precursors that, upon further polymerization reaction, form the walls of the microcapsules. Resin precursors used in this process include melamine-formaldehyde, urea-formaldehyde, and urea-melamine-formaldehyde polymers.
In the art of microencapsulation, the particle size and size distributions are controlled by mechanical shear, aqueous phase viscosity, and oil phase viscosity. The degree of shear and amount of shear energy produced depend significantly on the geometry of a particular shear device and residence time. For example, a higher shear rate and longer residence time would produce a finer microcapsule size. U.S. Pat. No. 5,643,506 describe a continuous process of generating microcapsules using a conventional LP. Gaulin colloid mill device.
Such a device is capable of generating a high shear rate by driving the conical motor at a very high rpm. The final microcapsule size is controlled by how fast the motor rotates, the viscosity of the oil and aqueous phases, and the ratio of the organic phase to aqueous phase. It is well known in the art that microcapsules generated by the above process have a broad size distribution and poor batch-to-batch reproducibility. There is a broad distribution of the shell thickness within the same batch of microcapsules especially when the shell forming materials are added to the oil phase. Larger particles have a thicker shell, and smaller particles have a thinner shell. This undoubtedly produces a distribution in the microcapsule permeability or the degree of impermeability.
When microcapsules are used in imaging systems such as carbonless paper or light sensitive pressure developable or heat developable image media, the microcapsule shell must be impermeable to the core materials. They must also have very low permeability to oxygen if the physical characteristics of the microcapsules are changed by free radical initiated reactions, since oxygen is an inhibitor. The microcapsule shell functions as a barrier material to prevent oxygen from infiltrating the light sensitive composition. Upon exposing the material to light, free radicals consume the oxygen present inside the capsule and the polymerization reaction proceeds. If the oxygen re-infiltrates the light sensitive composition, the photographic speed of the media is very poor.
Microcapsules need to be resistant to low pressure during normal storage and handling process, otherwise premature release of the core material will occur. In addition, microcapsules used for imaging applications need to be capable of withstanding temperatures up to 100° C. since during the manufacturing process the coating may be dried by heating. It is believed that the ability to control microcapsule size and size distribution is crucial to meet those requirements.
U.S. Pat. No. 4,842,978 describes a process for the preparation of light-sensitive microcapsules which comprises encapsulating silver halide and an ethylenically unsaturated polymerizable compound with a shell comprising an amino-aldehyde resin in an aqueous medium in the presence of an anionic protective colloid, wherein the anionic protective colloid is a mixture of pectin and a polymer comprising a repeating unit derived from stylenesulfonic acid, and the weight ratio of the pectin to the polymer ranges from 0.1 to 10.
There is still a need for microcapsule compositions having a narrow size distribution and good imaging capabilities.